This invention relates to an eccentric gear for providing a speed difference between two machine elements which are rotatably mounted in relation to a reference member and individually rotatable around a common geometrical symmetry axis, including a pair of co-operating gear rims, one of which is eccentric in relation to the symmetry axis and one of which is in the form of a male-like gear rim formed with out-turned teeth, and the other one is in the form of a female-like gear rim which is formed with in-turned teeth and has a larger diameter than the male gear rim in order to enable engagement of only one or a few of the teeth of the male gear rim in the female gear rim, one of the gear rims being arranged to roll off against the other one.
Prior Art
In comparison to planetary gears, eccentric gears have a plurality of fundamental advantages. Eccentric gears enable, above all, a high gear change in one single step, at the same time as the same have servo-technically favourable properties, such as limited play, large torsional stiffness as well as a low moment of inertia of the input shaft of the gear. Different forms of eccentric gears are previously known, e.g. by EP 345 276 (SE 8700291-1) and U.S. Pat. No. 5,269,202 (marketed under the trademark HARMONIC DRIVE). Other eccentric gears are commercially available under the trademark CYCLO DRIVE. These previously known constructions generally enable gear changes within the range of approx. 10:1 to approx 250:1.
The previously known reduction gears of the eccentric type are, however, also associated with problems and shortcomings. Among other things, a large number of teeth on the male and female gear rims of the gear is required at high gear changes, which entails that the teeth compulsorily become small; something which limits the performance of the gears and increases the manufacturing difficulties. As an example, it should be mentioned that gears of the type HARMONIC DRIVE require approx 500 teeth for the gear change of 250: 1, while other constructions may be formed with fewer teeth, e.g. down to approx. 250 teeth. Another problem, common for a plurality of the previously known eccentric gears, is the so-called xe2x80x9ctransmitterxe2x80x9d-function, i.e. the need for particular means to eliminate the eccentric motion between the eccentrically working gear rim and the output shaft of the gear. Proposals for solutions of this problem are found, for instance, in the following patents: EP 594 549 (SE 9203101-2) and EP 791 147 (SE 9404154-8). Strength problems, friction losses and angle plays increasing with time do, however, characterize the constructions according to the last-mentioned patents, the means for eliminating the eccentric motion between the gear rim and the output shaft tending to limit the performance of the gears within given outer dimensions. Furthermore, these means account for a considerable part of the total manufacturing cost of the gears.
The present invention aims at obviating the above-mentioned shortcomings of previously known eccentric gears and at providing an improved eccentric gear. Therefore, a primary object of the invention is to provide an eccentric gear which with few, simple components can bring about very high gear changes while using a comparatively limited number of teeth in the co-operating gear rims. A further object is to provide an eccentric gear, the gear change of which may, in a simple and flexible way, be chosen within a wide spectrum of gear changes, all from very moderate gear changes (e.g. the magnitude of 10:1) to extremely high gear changes (the magnitude of 30 000:1 or more). By constructive simplicity, the gear should, further-more, be possible to manufacture in an effective way production-wise and at low costs.
According to the invention, at least the primary object is attained by the features defined in the characterizing clause of claim 1. Preferred embodiments of the eccentric gear according to the invention are furthermore defined in the dependent claims.